CHICO — A Chico couple were arrested today for alleged ongoing abuse of their 2-year-old adopted son, with the mother killing him by inflicting blunt force trauma to his head in April.

Nurse Edelyn Yhip, 43, was arrested on suspicion of murder, felony child endangerment and abuse of a child younger than 8 years old causing death said, Sgt. Jason Hail of the Butte County Sheriff’s Office.

Chico cardiologist Dr. James Yhip, 46, was also arrested on suspicion of murder and felony child endangerment, Hail said.

About 6 p.m. April 18, Edelyn Yhip called 9-1-1 from a home on the 4500 block Gardenbrook Drive to report that her son, Benjamin, was unresponsive, Hail said.

Medical personnel responded and transported the boy by ambulance to Enloe Medical Center. He was stabilized in the emergency room, then an air ambulance took him to Sutter Memorial Hospital in Sacramento where he deteriorated and died the next afternoon.

The cause of death was determined to be blunt force trauma to the head. Butte County District Attorney Mike Ramsey said investigators are unsure how the injury was inflicted, but it was massive enough to cause a brain injury but not a broken skull.

After a lengthy review of medical and adoption records, investigators determined that the parents allegedly abused the boy and the mother was responsible for his injuries April 18, Hail said. Some of the records were in Chinese. The Yhips adopted Benjamin and his twin fraternal brother from Taiwan, Ramsey said.

They also separately adopted a daughter, Hail said. The other two were placed with Children’s Services after Benjamin’s death.

“The other children appeared to be fine,” Ramsey said. “He appears to be the targeted child.”

Ramsey said investigators think Edelyn Yhip hurt the boy that day, however they do not discount the possibility that James Yhip could have been involved.

The boy’s medical problems at first looked like they were disease-based, but Ramsey said it appears they resulted from abuse and active neglect by both parents. He had been hospitalized once for malnutrition and had broken bones from previous injuries.

“This child had been gravely abused over his time with the Yhips,” Ramsey alleged.

Ramsey said he is not sure why the parents would hurt the toddler. The boy had behavioral problems, which could have been innate or as a result from the alleged abuse.

“It’s a very sad, very tragic case and there was some opportunity during the process for some red flags,” Ramsey said. “There was hospitalization at Sutter Memorial at the end of November for extreme malnutrition. It was probably explained away.

“I think as a doctor and Mrs. Yhip being a medical professional they were given more of a break than perhaps someone else might have been.”

A Sutter Memorial Hospital representative could not be reached by deadline Wednesday.

Both Yhips work in Enloe Medical Center, James as a cardiologist since 2006 and Edelyn as a nurse in the medical oncology clinic since 2011, hospital spokeswoman Khristina Chavira said. The cardiologist also worked at Northstate Cardiology Consultants.

Chavira said the pair did not treat children while working in the hospital.

“We are deeply saddened and distressed by this terrible event and hope for a fair and speedy resolution to the judicial process,” an Enloe press release read. “In the meantime, we are committed to respecting the privacy of the Yhip family and to cooperating fully in the investigation as the sheriff’s office deems appropriate.”

The Butte County Sheriff’s Office and Butte County District Attorney investigators worked together on the case.

The Yhips were still in the Butte County Jail as of 5 p.m. today, Ramsey said. The father’s bail was set at $1.05 million. The mother’s bail was set at $2.05 million. If they don’t bail out of jail, they might be arraigned Friday.